A polybutadiene rubber for use in golf balls as a base material rubber is generally required to have high rebound resilience and excellent processability. A higher Mooney viscosity improves rebound resilience but deteriorates processability. A wider molecular weight distribution improves processability but deteriorates repulsion. Thus, there is a tradeoff therebetween. For the purpose of making processability and rebound resilience compatible, improvements in polybutadiene rubbers have been tried and various proposals have been made.
For example, Patent Documents 1 and 2 disclose polybutadiene rubbers synthesized in the presence of a Ni-based catalyst and having a higher Mooney viscosity and a wider molecular weight distribution, and try improvements in durability and rebound resilience. Patent Document 3 discloses a polybutadiene rubber with a cobalt catalyst and having a higher Mooney viscosity, which improves rubber rebound resilience and dimension stability. Patent Document 4 discloses a polybutadiene rubber with a cobalt catalyst and having a lower Mooney viscosity, which improves rubber rebound resilience and processability. Patent Documents 5, 6, and 7 disclose methods of improving rubber rebound resilience and processability by blending a polybutadiene rubber having a lower Mooney viscosity and a polybutadiene rubber having a higher Mooney viscosity with a cobalt catalyst.    Patent Document 1: JP-A 63-275356    Patent Document 2: JP-A 2-177973    Patent Document 3: JP-A 2004-263094    Patent Document 4: JP-A 2004-292667    Patent Document 5: JP-B 6-80123    Patent Document 6: JP-A 2002-143348    Patent Document 7: JP-A 2003-154033